It is known to provide a test instrument, such as an oscilloscope, having an input terminal connected to an attenuator, with selectable high impedance and low impedance input terminations for use when testing high and low voltage sources respectively. The attenuator may be a hybrid circuit formed on a ceramic substrate, in which case the terminations may be provided by respective load resistors which are screened on the ceramic substrate by conventional thick film techniques. Typically, the low impedance termination is provided by a 50 ohm resistor designed to dissipate about 1 watt, in which case the maximum RMS voltage rating for the 50 ohm input mode may be 5 volts, equivalent to a power dissipation of 0.5 watt.
The high impedance input termination may be a 1 megohm resistor. If the instrument is being used in the 1 megohm input mode for measuring a high voltage, and is switched to the 50 ohm input mode preparatory to measuring a much lower voltage without first disconnecting the high voltage, excessive power may be applied to the 50 ohm resistor, causing the resistor to burn out. This problem has previously been avoided by using an integrated circuit to sense the voltage drop across the 50 ohm resistor. Since the power dissipated in the resistor is proportional to the RMS value of the voltage drop across the resistor, the circuit must be capable of effecting an RMS conversion. An appropriate integrated circuit is quite complex and expensive, since it must be capable of operating over the entire bandwidth within which the instrument is designed to operate, and should be capable of carrying out an RMS conversion not only on simple sine waves but also on arbitrary waveforms. Moreover, the IC may itself degrade the shape of the waveform reaching the attenuator.